The present invention relates to a reflow soldering method and a reflow soldering apparatus for realizing the same using a lead-free solder alloy which is low in toxicity but poor in wettability in general. The lead-free solder alloy can be applied to connection of a target component such as an electronic component to a circuit substrate such as an organic substrate, and is an alternative to an Sn-37Pb (unit: % by mass) solder used for soldering at around 220° C.
However, the lead-free solder alloy is generally poor in wettability as compared to the conventional Sn-37Pb solder, and especially in the case of an Sn—Zn solder, wetting of the solder is hardly spread on a connection member. As a result, stress is concentrated at a part where wetting is comparatively less spread, and the part becomes an early generation point of cracks, thus leading to deterioration in connection reliability in some cases.
In order to improve the wettability, the melting property of the solder is improved by increasing the fluidity of the solder, that is, the surface tension or kinetic viscosity of the solder at the time of melting is lowered.
As a method of solving the above-described problem, Patent Document 1 which is a previous application proposes a method in which ultrasonic oscillation is applied through a circuit substrate, and a melted solder is indirectly oscillated with high energy, though the oscillation is of minute amplitude, so that a void derived from an outgas of a flux (if the supplying amount of a flux is large, a gas remains in the solder) of a solder connection portion is discharged, or non-connection of melted solder bumps and a melted solder paste is prevented on a substrate basis.
Further, as a soldering method by locally heating only a solder connection portion or its surrounding portion, which is the main subject of the present invention, there is a method called as a bump repair in which in the case where an electronic component having solder bumps such as a BGA (Ball Grid Array) is poorly connected by reflow soldering, a solder connection portion of the electronic component is heated by hot air, the solder is melted to remove the electronic component, and the electronic component is soldered again.